Amplification system



Sept. 11, 1934, G. L. BEERS 1,973,037

AuPLIFIcA'rIoN SYSTEM Original Filed Nov. 30, 1926 2 Sheets-Sheet l 5 InB E (t l w U v 2:

6K) 800 I000 I200 I400 I500 WITNESSES: Fre uenc Kiloc des INVENTORGeorge L. Beers.

Paten .1! Sept. 11, 1934 AMPLIFICATION SYSTEM George L. Beers,Merchantville, N. .12, assignor to Westinghouse Electric & ManufacturingCompany, a corporation of Pennsylvania Original application November30,1926, Serial No. 151,722. .Divided and this application February 1,1932, Serial No. 590,163

12 Claims. (Ci. 175-359) My invention relates to amplification systems,and it has particular relation to a system designed for voltageamplificatiomatradio frequencies.

This application is a division of my application Serial No. 151,722filed November 30, 1926 now Patent 1,907,478; May 9, 1933.

One object of my invention is to provide a coupling means for improvingthe selectivity characteristics of a radio-frequency amplifying system.i

Another object of my invention is to improve the voltage amplificationcharacteristics of a radio-frequency amplifying system.

Another object of my invention is to provide an improved radio-frequencytransformer to obtain the above results.

Another object of my invention is to provide a radio-frequencyamplifying system that will be reasonably selective over the entirerange of frequencies now used in radio broad-casting, and, at the sametime, give a substantially equal voltage amplification over the samerange.

Anotherobject of my invention is to provide a radio-frequencyamplification system that is inherently stable and does not tend tooscillate over the range of frequencies for which it is designed.

Another object of my invention is to provide a coupling transformer theprimary of which will have a resonant period substantially outside ofthe frequency band within which, it is intended to function.

Other objects of my invention will become apparent from the followingdescription of my invention.

In the construction of radio-frequency amplifiers according to theteachings of the Alexanderson Patent No. 1,173,079, it has been found,by experiment, that the inductance of theprimaries of the tunedtransformers should be kept low in order to lessen the tendency of theamplifier to go into self-oscillation. It is customary to so proportionthe primaries of such tuned transformers that the natural period thereofis somewhat higher than any frequency to which the secondary is intendedto be tuned, with the result that, although the amplification at thehigher frequencies is satisfactory, the selectivity, by reason ofprimary resonance and high losses in the secondary, is less good at thehigh-fre quency end of the tuning range than at the low-frequency end.

On the other hand, a radio-frequency transformer of the usual type,having a low-.induc tance primary, is extremely selective at thelower-frequency end of the range, since the radio-.

frequency resistance of the secondary is many In order to eflicientlyamplify both side bands 05 I resulting from modulation by voice andmusic, it is necessary that the resonance curve, at any givenradio-frequency over the tuning range, shall be from 8 to 10 kilocycleswide at or 90% of the peak or resonance-current.value.

With radio-frequency transformers of the usual type, input frequenciesone, or one and one half kilocycles above or below the frequency towhich the secondary may be tuned at the low-frequency end of the rangewill give ap- 75 proximately 90% of the resonance current, while, at thehigh-frequency end of the range, the input frequencies may vary as muchas fifty kilocycles above or below resonance and still give secondarycurrents 90% of the resonance current. The selectivity at highfrequencies may be somewhat improved by using several stages, but theimprovement is not sufficient to be satisfactory. Even a single-stageamplifier is much too selective for good quality reproduction at lowfrequencies and this condition is greatly exaggerated in a multi-stageamplifier.

If, therefore, an attempt is made to operate a receiving set of theusual multi-stage, tuned radio-frequency type in the vicinity of ahighpowered broadcasting station operating on a frequency which liestoward the high-frequency end of the tuning range, much difiiculty isexperienced in receiving signals from stations opthe inductance anddistributed capacity of the primary of each inter-tube, radio-frequencytransformer that it has a natural period considerably below the lowestfrequency to which the secondary is intended to be tuned. In addition, Imay so relatively arrange the primary and secondary inductors that, athigh frequencies, the inductive transfer of energy therebetween isreinforced by energy transferred through the capacity existing betweensuch inductors by reason of their space relationship.

The novel features which I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof will best beunderstood by reference to the following description, taken inconnection with the accompanying drawings, in which:

Figure 1 is a diagrammatic view. of a preferred form of my inventionincorporated into a multi-stage radio-frequency amplifying system,

Fig. 2 is a sectional view of a preferred form of inter-tubetransformer, and

Fig. '3 is a diagram illustrating graphically the difference between thevoltage amplification characteristics of my improved coupling system andsystems of the prior art,

Fig. i is a diagram showing curves illustrative of the selectivitycharacteristics of the usual low-inductance primary radio-frequencytransformer, and

Fig. 5 is a diagram showing curves illustrative of the selectivitycharacteristics of my improved transformer.

Referring specifically to Fig. 1, a thermionic device 1 having afilament 2, a grid 3 and a plate is provided with an input circuitcomprising an inductor 5 shunted by a tuning condenser 6. The inductor 5may be a loop, or it may be a coupling inductor of any well known type,associated with an antenna-ground circuit (not shown). One end of theinductor 5 is-connected to the grid by a conductor 7; a conductor 8leads from an intermediate portion of the inductor 5 through a gridbiasing battery 9 to the filament 2, and the opposite end is connectedto the plate 4 through a small variable or fixed condenser 11; Thepurpose of the intermediate tap and condenser, in radio-frequencyamphfiers of the usual type, is to prevent the generation ofself-oscillations in the tube and accompanying circuit network, as isexplained in the Rice Patent No. 1,334,118. In my system, however, thecondenser 11 has a somewhat differentfunction, which will be laterexplained. a

An inductor 12, which is the primary of a radio-frequency transformer13, is connected between the plate 4 and the filament-2, a battery 14supplying plate potential. A battery 15 in series with a rheostat 16supplies filament power, and the same battery also may be utilized toenergize the filaments of other tubes in the system, as indicated in thedrawings.

A. second thermionic device 18 having a filament 19, a grid 21 and aplate 22, has its grid 21 connected to one end of the secondary 23 ofthe radio-frequency transformer 13,- and its plate 22 connected to theother end thereof through a variable or fixed condenser 24. A condenser25 is connected across the inductor 23 in order that it maybe tuned to adesired frequency, and a conductor 26 leads froman intermediate pointthereon, through a grid-biasing battery 27, to the filament 19.

Plate potential for the thermionic device is supplied from the battery14, the primary 28 of a radio-frequency transformer 29 being connectedbetween the battery and the plate. The secondary 31 of the transformer29 is shunted by a tuning condenser 32 and is connected to the remainingportion of the system by conductors 33 and 34.

The two tuned radio-frequency amplifying stages may be followed byadditional radiofrequency stages, or by a detector and audiofrequencyamplifier. The additional apparatus is optional, and, since it mayfollow conventional lines, if desirable, is merely indicated by arectangle 35.

In order that the inter-tube transformers be sumciently selective toreasonably eliminate interfering frequencies, it is only necessary thatthe ratio of reactance to radio-frequency resistance of the secondary bemade at least so high that primary frequencies five kiiocycles on eachside of resonance do not induce currents of more than of the resonancevalue in the secondary.

With the usual radio-frequency transformers, having low-inductanceprimaries, the approach toward resonance conditions in the primary atthe higher frequencies effectively adds resistance to the secondary atsuch frequencies. Inasmuch as the radio-frequency resistance of thesecondary is also higher at high than at low frequencies, it is seenthatthe total damping at high frequencies is considerably greater thanit is toward the low-frequency end of the scale, and the selectivity isconsequently poorer.

The results of a large number of tests on radio-frequency transformersof the usual type are summarized by the curves in'Fig. 4. Curve Crepresents the voltage across the secondary tuned to 600 kilocycles,with varying input frequencies, and it is significant to note that afrequency five kilocycles away from resonance gives but 50% of theresonance voltage. Curve D illustrates the fact that the high-frequencyend of the range in the usueltransformer is much less selective than atthelow-frequency end, frequencies five kilocycles away from resonance at1450 k'ilocycles giving approximately 95% of the resonance voltage.

In order, therefore, to improve the selectivity at high frequencies, itis necemary to greatly reduce the losses at such frequencies. Theselosses may be reduced, according to my invention, by giving to theprimary a resonance frequency such that itcannot be near resonance atthe high frequencies. This may-be accomplished by either greatlyincreasing or greatly decreasing the resonance frequency of the primarywith respect to highest frequency to which the secondary is tunable; If,however, the resonance frequency of the primary is increasedby loweringits inductance, the amplification at the low frequencies is poor.

Therefore, -1 preferably lower the resonance frequency of the primary toa point -near or below the lowest frequency to which the secondary istunable. Such a primary does not increase the efiective resistance ofthe secondary at high frequencies but does increase the effectiveresistance thereof at low frequencies. The effective resistance thusintroduced compensates the decreased damping resulting from thedecreased radio-frequency resistance of the secondary at 1,978,087 lowerfrequencies and broadens-the tuning atsuch frequencies.

Curves E and F, in Fig. 5, summarize the results of a large number oftests made on my improved transformer. It will be noted from thesecurves that the selectivity at both the high-frequency end and thelow-frequency end of the range is substantially the same, the relativedisplacement between the two curves never becoming greater than onetenth of the maximum resonant voltage.

By properly choosing the inductance and distributed capacity of theprimary, and by carefully adjusting the mutual inductance of the primaryand secondary, I am thus able to construct a transformer that will besatisfactorily selective at both the high and low-frequency ends of thetuning range.

In order to cover the present broad-cast-frequency range, I have foundthat a transformer having a primary inductance of approximately 5000micro-henries and having a natural period of approximately 400kilocycles, when in circuit, coupled with a secondary inductance of 175micro-henries, is quite satisfactory. When using a transformer of thesedimensions, the tuning condenser shunting the secondary should have amaximum capacity of 550 micro-microfarads and a minimum capacity of15-20 micro-microfarads or less.

Referring particularly to Fig. 2, my preferred transformer comprises aprimary 40 of 1250 turns of #30 double-cotton-covered wire, in the formof a duo-lateral coil, and a secondary 41 of 72 turns of #24double-,cotton-covered wire wound as a single-layer solenoid. The insidediameter of both primary and secondary is two inches and they arepreferably separated to inches axially.

The curve A in Fig. 3 illustrates graphically the voltage-amplificationcharacteristics of a transformer constructed according to my invention,while the curve B illustrates the characteristics of a transformerhaving the usual low-inductance primary.

According to my invention, therefore, highervoltage amplification may beobtained at the lower frequencies than with transformers havingprimaries of relatively low inductance. This condition is considereddesirable, inasmuch as the better broadcasting stations are, at persent,operating at frequencies toward the lower end of the broadcasting range.

An amplifying system constructed according to my invention is relativelystable. At the higher frequencies, the reactance of the highinductanceprimaries in the plate circuits of the tubes is predominately capacitiveand, consequently, the conditions for negative feed-back are present. Inorder, therefore, that the systemshall amplify efiiciently, a certainamount of positive feed-back should be supplied and this feed-back ismost conveniently introduced by utilizing a small condenser between theplate electrode and one end of the inductor associated with the grid ofthe same tube.

As already mentioned, the action of this arrangement is different fromthat of the Rice Patent 1,334,118, although diagrammatically they aresimilar in appearance. In the Rice patent, the neutralizing condenserand that portionof the grid inductor included between the connectionfrom the filament and the connection to the condenser cooperate toplace, on the grid of the tube, potentials equal and opposite in phaseto potentials placed thereon by reason of the tube-capacity couplingbetween the input and the output circuits in order that regenerativefeed-back shall be compensated. In my invention on the other-hand, thenormal reactance of the plate circuit is capacitive, tending towardde-generation", instead of toward regeneration and oscillation, and thecondenser 11 in combination with the portion of the winding with whichit is connected, act to produce a regenerative feed-back. While,therefore, the circuit arrangement is diagrammatically like that of theRice patent, the effect of the condenser and its associated coil isregenerative after the well known Weagant regenerative circuit.

Although not specifically shown in the drawings, it is believed obviousthat my invention may be applied to systems in which the plate or outputcoil for each stage is inductively associated with another coil that iscomprised in a feedback system. The feed-back coil may be a continuationof the output coil itself, in which case the said output coil could becharacterized as split".

I have found, in addition, that the necessary amount of positivefeed-back may be supplied by utilizing certain other well knownfeed-back systems, in which either capacitive or inductiveback-coupling, or a combination of the two, is

the respective windings so they are adjacent,

provided the relative direction of the windings is such that thecapacity coupling is in proper phase.

By varying the spacing between the primary and these secondary as byshifting the primary winding on the coil form 42 toward or away from thesecondary winding, the voltage-amplification characteristics of thetransformer may be appreciably altered, inasmuch as there is a consid--erable transfer of energy at the higher frequencies across theinter-winding capacity. An optimum spacing can be determined for eachtransformer, the spacing 42 for the specific transformer illustrated inFig. 2 being approximately inches.

By reversing the relative direction of the windings, the capacitycoupling will then oppose the electromagnetic coupling, and theamplification at the higher frequencies will be considerably lessened.

I have accordingly provided a radio-frequencyamplification system whichhas a substantially straight-line selectivity-characteristic over therange of frequencies for which it is designed. My improved couplingsystem may also be given an approximately straight-linevoltage-amplification characteristic, although it is somewhat preferableto have a higher voltage step-up at the lower frequencies.

My improved system is also remarkably free from parasitic oscillations,and, although preferably utilizing a feed-back means analogous to theusual neutralizing condenser and associated network, such means is notprimarily for th pm.

lations.

Although I have illustrated and described only one specific embodimentof my invention, it is believed obvious that numerous modifications willbe apparent to those skilled in the art. My invention, therefore, is notto be limited except insofar as is necessitated by the prior art and bythe spirit of the appended claims.

I claim as my invention: l. A radio-frequency transformer comprising asingle-layer solenoid having a predetermined tunable frequency range,and a multi-layer coil co-axial therewith and totally external thereof.

2. A radio-frequency transformer having a secondary. comprising asingle-layer solenoid, and a primary comprising a concentratedinductance co-axial therewith and totally external thereof, the naturalperiod of said primary being low in comparison with the averagefrequency which the transformer is designed to handle.

3. A radio-frequency transformer having a secondary comprising asingle-layer solenoid, and a primary comprising a concentratedinductance co-axially therewith, the natural period of said primarybeing low in comparison with the average frequency which the transformeris designed to handle.

4. A radio-frequency transformer having a primary winding, and asecondary winding coupled thereto, said primary winding having a greaterinductance than said secondary winding, said windings being wound withrespect to each other to constitute a capacity coupling between the two.

5. A radio frequency transformer comprising a primary winding and asecondary winding,

said secondary having a predetermined tunable frequency range, saidprimary having a natural period below the lowest frequency of saidfrequency range, said windings being so disposed with respect to eachother as to constitute a capacity coupling between them.

' 6. A radio-frequency transformer comprising a helical-wound secondarywinding having a length at least as great as' its diameter, and aprimary winding of smallamal length relative to that of said secondarywinding located adjacent one end of said secondary winding, said primarywinding comprising a larger number of turns than said secondary windingwhereby a circuit in which said primary winding is connected is renderedresonant at a frequency which is below the range of frequency to bereceived by said circuit.

7..A radio-frequency transformer according to claim 6 in which'saidprimary coil is located at the low-potential end of said secondary coiland the high-potential end of said primary coil is at its outercircumference, whereby the capacitive coupling between the windings isnegligible.

8. A high-frequency transformer comprising a helical-wound secondarycoil and a closely wound multi-layer primary coil, said primary coilbeing located near the high-potential end of said secondary coil, theportion of said primary coil which is nearest the secondary coil beingthe high-potential portion, whereby the 9:; coupling between said coilsis substantially capacitive as well as magnetic.

9. A high-frequency transformer according to claim 8 in which theefiective direction of said primary winding is opposite that of saidsecond- 131". ary winding, whereby said capacitive coupling aids saidmagnetic coupling.

10. A high-frequency transformer according to claim 8 in which saidprimary coil is effectively wound in the same direction as saidsecondary winding, whereby said capacitive coupling opposes saidmagneticcoupling.

11. A coupling system comprising two windings, the first of saidwindings being a cylindrical coil having an axial length at least asgreat as its diameter, the second of said windings being amulti-layercoil having an axial length oo-axial therewith and totallyexternal thereof,

said coils being spacially adjustable relative to each other.

' GEORGE L. BEES.

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